1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for rapidly assigning traffic channels in digital wireless communications rapidly assigning traffic channels in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or xe2x80x9cmobile stationsxe2x80x9d and a fixed network infrastructure. Typically, the mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In CDMA communication systems, transmissions from a base station to a mobile station are referred to as xe2x80x9cforward linkxe2x80x9d transmissions. Transmissions from a mobile station to a base station are referred to as xe2x80x9creverse linkxe2x80x9d transmissions. The basic radio system parameters and call processing procedures for exemplary prior art CDMA systems is described by the TIA specification which is entitled xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as xe2x80x9cIS-95xe2x80x9d.
Both voice and data services are available using CDMA communication systems made in accordance with IS-95. However, disadvantageously, data calls use the same airlink protocols, traffic channels, physical layers, signaling methods, call processing schemes and airlink protocols are used by the voice calls. While the prior art call processing schemes and signaling methods are efficient and effective for voice services, they are inefficient for data services, especially when the data services comprise very short duration calls. As is described in more detail below, it can take between two and three seconds to establish or xe2x80x9csetupxe2x80x9d an average voice traffic channel using the prior art call processing schemes. While this setup time may be acceptable for a voice call that, on the average, may have a duration of between 100 and 300 seconds, it is unacceptable for a data call having a duration of only a few seconds, or less. Therefore, an improved technique is needed for assigning data traffic channels in a CDMA communication system. The causes of traffic channel assignment delays in the prior art systems become apparent by reviewing CDMA call flow examples. Therefore typical prior art CDMA call flow examples are now described.
CDMA Call Flow Examples
Table 1 shows a simple call flow example as set forth in IS-95. Table 1 uses the following conventions:
All messages are received without error.
Receipt of messages is not shown (except in the handoff examples).
Acknowledgements are not shown.
Optional authentication procedures are not shown.
Optional private long code transitions are not shown.
Table 1 shows a simple call flow example wherein a mobile station originates a call. Base station originated calls follow similar procedures. Messages are transmitted from the mobile station to the base station using the access channel. Messages are transmitted from the base station to the mobile station using the paging channel. As shown in Table 1, the mobile station first detects a user-initiated call, and then sends an xe2x80x9coriginationxe2x80x9d message via the CDMA access channel. The access channel is a slotted random access channel. The mobile station transmits on the access channel using a random access procedure. Many parameters of the random access procedure are supplied by the base station in an access parameters message. The entire process of transmitting one message and receiving (or failing to receive) an acknowledgement for that message is called an xe2x80x9caccess attempt.xe2x80x9d Each transmission in the access attempt is called an xe2x80x9caccess probe.xe2x80x9d Within an access attempt, access probes are grouped into access probe sequences. Each access probe sequence comprises a fixed number of access probes. The first access probe of each access probe sequence is transmitted at a specified power level relative to the nominal open loop power level. Each subsequent access probe is transmitted at a power level that is a specified amount higher than the previous access probe.
In normal CDMA operation, when a mobile station user initiates a phone call, the mobile station sends an access probe to the base station. If the access probe is properly received by the base station, the mobile station should receive back an acknowledgement from the base station. Once the acknowledgement is received by the mobile station, the mobile station is instructed by the base station to wait and to stop sending further access probes to the base station. This is necessary because access probes produce interference on the communication channel. The mobile station therefore waits until it is assigned a traffic channel by the base station. The base station then communicates this request for a traffic channel and information about the mobile station to a base station controller (BSC). The BSC performs several administrative functions, possibly including authenticating the mobile station. The BSC then reviews the pool of available resources and allocates an element for the requesting mobile station.
As shown in Table 1, the base station informs the mobile station of the traffic channel assignment by sending a channel assignment message via the paging channel. Once the mobile station receives its channel assignment from the base station, it changes its receive and transmit frequencies, in addition to other relevant parameters, to the assigned traffic channel. The mobile station then attempts to initiate communication on the assigned traffic channel by establishing or xe2x80x9csetting upxe2x80x9d the traffic channel. If the traffic channel initialization is successful, the mobile station then acquires the traffic channel. The mobile station then begins sending a preamble on the reverse traffic channel to allow the base station to acquire the mobile station. As shown in Table 1, the base station acquires the reverse traffic channel and sends a base station acknowledgement order to the mobile station if the reverse traffic channel was properly acquired. At this point the mobile station and the base station begin negotiating service. The communication link can fail at any point during this negotiation process. However, if the negotiation process is successful, communication commences and a telephone conversation ensues. If the mobile station receives more than one base station, it may then request the allocation of additional traffic channels from the other base stations.
The prior art traffic channel assignment procedures shown in Table 1 take a relatively long period of time to execute. For example, from the time a base station receives a traffic channel request from a mobile station via the access channel, it typically takes between two and three seconds before a traffic channel is assigned and a base station acknowledgment order is transmitted to the mobile station. As noted above, this service delay is acceptable for voice services wherein the duration of voice calls are typically between 100 and 300 seconds. However, this service delay is unacceptable for data services wherein the duration of data calls are typically only a few seconds, or less. In addition, the assignment of traffic channels utilizes scarce system resources such as specific base station hardware, limited numbers of code channels, and transmission bandwidth (which is required both for tracking and for power control even when no data is transmitted). Therefore, to improve system capacity and throughput, it is advantageous to rapidly de-assign traffic channels whenever the user terminal goes dormant. That is, whenever the user terminal and the base station no longer have information to exchange, it is desirable to rapidly de-assign the traffic channel associated with the mobile station and to quickly re-assign the traffic channel when more data is presented for transmission.
In addition to delaying service to the user (whether the service be voice or data based), delays associated with the assignment of traffic channels create further delays in providing power control of the user terminal (typically a cellular telephone). Because the user terminal""s transmission power can vary greatly, it is important to control the power of the user terminal as quickly as possible to avoid unnecessary co-channel interference that can both reduce system capacity and result in loss of the traffic channel. Therefore, it is desirable to both reduce the delays associated with the assignment of traffic channels and supervise the user terminals as quickly as possible. The present invention provides a method and apparatus that address these needs by rapidly assigning traffic channels to mobile stations in a wireless communication system. The present invention also provides a mechanism for rapidly and efficiently controlling the transmission power of the requesting mobile stations.
The present invention is a novel method and apparatus for rapidly assigning traffic channels in a wireless high-speed packet data communication system. The method and apparatus uses an access probe comprising a pilot preamble, a traffic channel request, and a pilot/data request channel (DRC) field. The access probe is transmitted to a selected base station via a reverse link access channel whenever a mobile station initiates a traffic channel assignment request. The mobile stations randomly select the access channels. The access probe is masked using a long code cover equal to an access channel cover code. All mobile stations use the same access channel cover when transmitting on a selected access channel. The mobile station transmits a sequence of access probes of increasing power until the access attempt is either successful or terminates. The mobile station monitors the forward link control channel and the forward link traffic channel while it is transmitting access probes to the base station.
The pilot preamble of the access probe allows the selected base station to easily detect the access probe transmission. In accordance with the present inventive method and apparatus, the mobile station transmits the traffic channel request immediately following the transmission of the pilot preamble. The traffic channel request includes data that identifies the requesting mobile station to the base station. Typically, this identifying data comprises an MSI that was previously assigned to the mobile station when it registered with the wireless packet data system. In addition to transmitting its MSI, the mobile station also transmits data that identifies the signal strengths and identities of all other base stations having received signal strengths exceeding a pre-determined threshold. Immediately after the mobile station transmits the traffic channel request it can begin using the reverse link traffic channel to transmit useful data to the selected base station. In one embodiment, the mobile station transmits the pilot/DRC field to the best base station it receives (i.e., the base station with the strongest signal received by the mobile station). The DRC includes traffic channel data rate information and is used by the mobile station to request the maximum data rate that it can reliably demodulate. The mobile station continues to transmit the pilot/DRC field for a period defined by the access probe tail.
Rather than waiting for the base station to authenticate and to assign a traffic channel to the mobile station, the mobile station begins communication on the traffic channel (identified by its MSI) immediately after transmission of the access probe. In essence, the traffic channels are pre-assigned to the mobile stations. In addition to speeding the assignment of traffic channels in a wireless packet data communication system, the present method and apparatus also allows base stations to begin supervising the transmission power levels of the mobile station immediately after the transmission of the access probe. In one embodiment, the mobile stations select from a group of available power control sub-channels. The mobile station uses the selected power control sub-channel when it begins to transmit data on the reverse communication link. The base station subsequently associates the mobile""s MSI with the selected power control sub-channel. The mobile station thereafter monitors the forward channel and determines whether its MSI is associated with the power control sub-channel it previously selected.
By enabling fast power level supervision by the base station, the potential interference that could have been caused by a rogue or uncontrolled mobile station is drastically reduced. In addition, by speeding the traffic channel assignment process, the present inventive method and apparatus facilitates short duration data calls, increases system capacity and throughput, and reduces the system costs associated with dormant mobile stations. Another embodiment of the present invention reduces the randomness of the channel selection process and thereby reduces the chance of collisions. In accordance with this alternative embodiment, the base station advertises the identities of available traffic channels (and available power control sub-channels) via the forward link control channel. In accordance with this embodiment, rather than randomly selecting a traffic channel based on its MSI, the mobile station selects an available channel (and associated power control sub-channel) from the available channel list advertised by the base station. After selecting the available channel and power control sub-channel, the mobile station initiates the channel assignment process using the access probe described in the first embodiment. Another alternative embodiment is described wherein the base station, and not the mobile station, initiates a traffic channel request. This embodiment is used when the base station has data identified for a selected mobile station. In accordance with this embodiment, whenever a base station has data identified for a particular mobile station that is currently not connected to the base station, a base station controller directs all base stations within the selected mobile station""s paging radius to transmit xe2x80x9cpagexe2x80x9d messages to the mobile station over the forward link. The base stations use the mobile station""s MSI to identify the page messages as directed to a particular mobile station. The mobile station continuously monitors the control channel and responds to pages addressed to its associated MSI. When the mobile station detects pages addressed to it (i.e., pages containing its MSI), it uses one of the methods described above to complete the traffic channel assignment process.
In yet another alternative embodiment, the base station initiates a traffic channel assignment by advertising both the identity of the selected mobile station and an associated power control sub-channel over the forward link control channel. The mobile station continuously monitors the forward link control channel and detects pages containing its associated MSI. When the mobile station identifies its page it transmits a traffic channel request message as described above. However, the mobile station also immediately begins monitoring the power control sub-channel identified in the page message.
The details of the preferred and alternative embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.